I didn't understand why the son would threaten to cancel the cruise over this.

And that's the crux of it, isn't it? Either Son is so incredibly unreasonable and drama-tastic, and prone to spoiling his kids in everything that there is absolutely no way that LW was taken by surprise here... or he's not. And if he's not, then something else is going on, and LW didn't see fit to give us all the details.